Ferromagnetic variable highfrequency inductor



Feb. 22, 19 9- w. J. POLYDOROFF FERRO MAGNETIC VARIABLE HIGH-FREQUENCY INDUCTOR Filed Sept. 2'7, 1944 I l l l 10 20 60 40 J0 MK'EEMEUI'AL pseMsns/a 17') l w w w 515 .5000 44 ford Joan /0 A5 Z0 Imp.

Patented Feb. 22,1949

FERROMAGNETIC VARIABLE HIGH- FREQUENCY INDUCTOR Wladimir J. Polydorofl, Glencoe, Ill.

Application September 27, 1944, Serial No. 556,062 In Canada July 15, 1944 The invention relates to ferromagnetic variable inductors suitable for their application at high frequencies, in which the variation of inductance is accomplished by new and novel means.

One object of the present invention is to make such variable inductors which can be eillciently operated at high frequencies, while covering sub-' stantial frequency range without any mechanical movement.

Another object of theinvention is to provide such inductors for high frequency circuits in which tuning is accomplished by variation of inductance through application of strong magnetic fields to said. inductor. It follows, therefore, that such an inductor may be used for remote control tuning of a high frequency circuit by means of a direct current circuit including electrical control elements.

Still another object of the invention is to provide such construction of ferroinductors in which magnetic fields are applied without affecting the operational characteristics of the inductors and associated high frequency circuits.

It has been known in the art that permeability of ferromagnetic materials is subject to change with magnetic density or with the intensity of magnetizing force applied to said materials.

It has been assumed that the magnetic materials suitable for high frequency operation may also alter their magnetic characteristics under different magnetizing conditions and the devices of this kind have been proposed without regard to their electrical characteristics. It was possible to somewhat change the inductance of a device having a ferroinductor with surrounding means of variable magnetization, but the losses of these devices were prohibitive to employ same in high frequency circuits. The present invention describes the improved methods embodying construction in which the aforesaid defects have been largely eliminated and it will be better un derstood if reference is made to the accompanying drawing in which:

Fig. 1 shows magnetic measurement curves for certain solid and comminuted magnetic materials.

Fig. 2 shows the incremental permeability or alternating current permeability obtainable in certain materials with superimposed steady magnetic flux.

Figs. 3 and 3a show a variable inductor present invention, 7

Fig. 4 shows operational characteristics of the device of Fig. 3, and

of the 8 Claims. (Cl. 171-242) Fig. 5 shows a modification of the inductor of Fig. 3.

Referring now to Fig. 1 the upper curve a shows the B-H curve for a solid magnetic material such as pure iron' and the curve b shows the same curve for a compressed powdered material. Because of the plurality of magnetic gaps, the magnetic induction for compressed core is but a small fraction of the induction obtainable with solid materials for the same mag netizing force.

A preferred magnetic material, may be one which compressed and then fused or sintered preferably in the direction of the magnetic field, as described in my U. S. Letters Patent 2,354,331,

behaves in accordance with curve 0 occupying a somewhat intermediate position between curves a and b. The ratio B/H termed as permeability is also plotted against magnetic induction. It can be observed that in all three materials the permeability is variable with B as shown by curves d, e, and f.

The present investigation of magnetic materials suitable for high frequency applications shows that these materials when tested in a permeameter by usual direct current methods have a certain value of initial permeability corresponding to the permeability obtained by alternating current bridge methods. The increase of magnetizing force beyond that of bridge measurements results in an increased permeability which may reach much higher values until further increase of magnetizing force results in lowering of permeability such as indicated by the curves e and f. If a high frequency material is subjected to a steady flux and additional alternating field is applied the resultant permeability is called incremental providing that alternating current flux is smaller than direct current flux. This incremental permeability may change con siderably under influence of steady flux so that if magnetic induction is varied the incremental permeability, as shown in Fig. 2 may change considerably. Fig. 2 shows a change of incremental permeability from 17 to 50 for a change of magnetic induction from 5 to 0 kilogauss as indicated by curve g for a powdered magnetic material and the change of incremental permeability is still more pronounced for sintered material as observed in curve h. This increase of change of incremental permeability because of superimposed D. C. flux is utilized in the present invention to provide a variable inductor suitable for high frequency operation.

A high frequency inductor was constructed of a low loss coil and a cylinder core which was inserted between the ends of a solid magnetic yoke through which strong magnetic fields were applied to the core. It has been observed that when such a yoke closes the magnetic circuit of a high frequency inductorthe losses become prohibitive and the Q (L) /(R) of such inductor may drop from 100 to zero when surrounded by magnetic yoke. It must be remembered that in order to pass strong magnetic fields, the magnetic yoke must be made of highlypermeable material and of sufllciently large cross section and mass. As the yoke is supplied with a magnetizing coll this coil finds itself in the common magnetic circuit further increasing the losses.

Fig. 3 shows a device of the present invention in which the above explained losses are largely eliminated.

A coil I which consists of a large number of turns is wound on a magnetic yoke 2 made of solid or laminated magnetic material of high permeability. Its cross section should be large enough to produce magnetizing force of the order of 100 cersteds without saturating the yoke itself. Between the ends 3 of the yoke a high frequency inductor 4 is placed having its winding 5 made in the low loss manner and its core material of such properties that the inductor has sufiiciently high Q to permit the successful operation of the circuit at high frequencies.

Theferroinductor of the present invention employs a closed high frequency magnetic circuit preferably in the form of two elongated cylinders 6, with ends 7 made also of high frequency magnetic material which construction substantially approaches a closed circuit for the high frequency inductor. If now the inductor isplaced between the ends of a solid or laminated yoke no appreciable loss of Q is observed. When direct current is absent in the windings the inductance 4 has a certain value of L and Q and in conjunction with a fixed capacitor it may form an oscillatory circuit responding to a certain frequency. When direct current isapplied to the winding I a magnetizing force is produced at the ends of the yoke and the steady magnetic flux flows through both cylinders of high frequency cores, causing the change of incremental permeability and the consequent change of inductance in the inductor 3.

The more current that is applied to coil I the i more magnetic induction is developed in the cores 6 and the permeability decreases in accordance with Fig. 2, until it reaches a practical minimum value which corresponds to magnetic induction of 5-6 kilogauss.

Since the magnetic induction B is equal to /a where is the flux and a is the cross section of high frequency core the smaller is the cross section, the larger will be B. Therefore, in order to produce large variation of incremental permeability the cross section of high frequency magnetic circuit should be very small compared with the cross section of a yoke and is best realized in the form of a magnetic circuit having two elongated cylinders of small cross section around which cylinders high frequency coils are closely wound, the cylinders being joined at both ends by the core pieces of high frequency core material.

While Figs. 1 and 2 show full values of permeability and incrementalpermeability, such full values may be only approached with closely wound toroidal coils in which the applications of mag.- netic fields presents certain difficulties. In the construction as shown in Fig. 3 the effective per..-

ineability of the order of 12-20 is realizable as,

compared with permeability of the material itself of the order of 50. However, the effective permeability is proportionalto permeability so that the range of variation of inductance is in the same ratio as the variation of permeability, providing the coils are Wound closely to the cores.

It may be interesting to observe the actual variation of inductance and frequency (if the inductor is bridged with a fixed capacitor) when direct current is applied to magnetizing coil I.

Fig, 4 shows this operational characteristic of the device of Fig. 3. Curve I shows the inductance variation when current in coil I is varied from 0 to 2 amperes and line 7c shows the relation between said current and the frequency obtainable with a fixed capacitor. This line It indicates that the frequency is proportional to the magnetizing current and if a device of the invention is placed for remote control operation two wires of any length may supply direct current to coil I from I battery through a variable resistance and an ammeter, which ammeter can be calibrated directly in frequency. The inductance variation is inversely proportional to the square root of said current if the cross sections of cores and yoke are properly chosen. The values of direct current shown in Fig. 4 correspond to the values of magnetizing force shown on Figs. 1 and 2 when said current flows through coil I. I

In the inductor of Fig. 3 a permissible Q is obtainable for successful operation at high frequencies. When the value of inductance is reduced by steady fiux the Q of the circuit generally improves, It is possible to construct the variable inductors so that the Q will remain constant with frequency changes.

Fig. 5 shows a modification of variable ferroinductor in which a closed pot type structure 8 is used as a high frequency inductor, the pot being also of elongated construction in order to obtain maximum variation in permeability.

While this second construction will produce less variation in permeability its Q value may be improvgd as compared with the Q of the circuit of Fig.

The above described ferroinductors may be used as part .of an oscillatory circuit either of series or parallel type or part of a high frequency oscillator or generator and can be remotely controlled by extending the wires from coil I to a remote source of direct current. It is possible to connect the magnetizing coil I to the anode of a thermionic tube in such a manner that the fluctuations in said anode current will affect the inductance of the inductor forming part of an oscillatory circuit. Thus an automatic regulation of either frequency or fidelity of a high frequency circuit may be secured. It is also possible to employ a variable inductor of the present invention for other purposes of high frequency and low frequency art where variable inductors or capacitors are now being employed. If the frequency of application decreases the permissible core permeability may be' increased, either by employing coarser grain of powdered material or by further sintering or by employing very thin magnetically soft laminations in the core of high frequency inductor which as before should be of the closed magnetic circuit type.

For the choice of material of the yoke it is important that the material should have high permeability and lowest possible coercivlty It has been found that a small residual magnetism left in the yoke after the employment of high current may somewhat offset the inductance of the device during the absence of magnetic flux.

Having thus described my invention what I claim is:

1. A high frequency variable inductor comprising a high frequency substantially closed magnetic circuit having an elongated main core portion and high frequency ferromagnetic closing sections, a high'permeability yoke made of magnetic material of low-coercivity, a magnetizing winding surrounding said yoke, the ends of said yoke embracing said ferromagnetic core to enable the passage of steady flux through said entire core in one direction while by-passing said winding and said yoke for high frequency flux and control means to supply direct current into said magnetizing winding to govern the amount of said steady flux, and a high frequency solenoidal coil wound on said elongated main core, said core having its length at least twice greater than the largest cross-sectional dimension of said core.

2. A high frequency Variable inductor comprising a coil, a high frequency magnetic core of the closed pot type including an elongated main core wound with solenoidal winding throughout its length and having a ratio of its length to its diameter of at least 2 to l, a, high permeability low coervicity yoke, a magnetizing winding surrounding said yoke, the ends of said yoke embracing said ferromagnetic core to enable the passage of steady flux through said core in one direction, while by-passing said winding and said yoke for high frequency flux, and control means in said magnetizing winding to vary the magnetic induction in said core.

3. A high frequency variable inductor comprising a pair of serially connected coils and a high frequency ferromagnetc core, said core being composed of two elongated .sintered cylinders having their length at least twice greater than their diameter joined together at their extremities by connecting pieces of same core material, a high permeability magnetic yoke, a magnetizing winding surrounding said magnetic yoke, the ends of said yoke engaging said pieces to enable the magnetic flux to pass through both of said cylinders in one direction and control means to vary the magnetizing current in said magnetizing winding.

4. A high frequency variable inductor comprising a coil and a high frequency core compressed of powdered material longitudinally sintered in the direction of magnetic flux, said core having substantially closed magnetic circuit comprising a main linear core portion and return portions, said main core portion having its length greater than its maximum cross-sectional dimension, a high permeability magnetic yoke of low coercivity, a magnetizing coil around said yoke, the ends of said yoke embracing said ferromagnetic core to enable the passage of steady flux through said entire core in one direction.

5. A high frequency variable inductor comprising a coil and high frequency sintered ferromagnetic core having a substantially closed magnetic circuit said core comprising a linear main portion and return portion the length of main portion being at least twice greater than its maximum cross-sectional dimension 9. high permeability yoke, a magnetizing winding surrounding said yoke, theends of said yoke embracing said ferromagnetic core to enable the passage of steady flux through said entire core in one direction and control means to alter said flux, the total cross section of said core being small in comparison with the cross section of said yoke.

6. A high frequency variable inductor comprising a coil and a high frequency ferromagnetic core having a substantially closed magnetic circuit, a high permeability yoke a magnetizing winding surrounding said yoke, the ends of said yoke embracing said ferromagnetic core to enable the passage of steady fiux through said entire core in one direction while by-passing said winding and said yoke for high frequency flux and control means to supply direct current into said magnetizing winding to govern the amount of said steady flux said core comprising an elongated main section having its length twice its maximum cross-sectional dimension and wound with a high frequency solenoidal winding which winding forms a portion of a circuit resonated at a desired frequency, said control means varying the inductance of said winding and frequency of said circuit and being provided with an ammeter directly calibrated in frequency.

'7. A high frequency variable inductor comprising a pair of serially connected coils and a high frequency ferromagnetic core, said core being' composed of two elongated cylinders joined together at their extremities by connecting pieces of the same core material the lengths of said cylinders being at least twice greater than their diameter, a high permeability magnetic yoke, a magnetizing winding surrounding said magnetic yoke, the ends of said yoke engaging said pieces to enable the magnetic flux to pass through both of said cylinders in one direction and control means to supply direct current into said magnetizing winding, the cross section of said ferromagnetic core being so proportioned to the cross section of magnetic yoke that the inductance variation is inversely proportional to square root of said current.

8. A high frequency variable inductor comprising a high frequency core composed of magnetic material characterized by large incremental permeability variations in response to variations of magnetic density, said ferromagnetic core having a substantially closed magnetic circuit said core having an elongated portion whose length is at least twice greater than its maximum cross-sectional dimension, a solenoidal coil wound around said portion, a high permeability magnetic yoke and means for subjecting said core to a magnetic induction whose magnitude is controllable by a variable direct current, the cross section of said ferromagnetic core being so proportioned to the cross section of magnetic yoke that the inductance variation is inversely proportional to the square root of said current.

WLADIMIR J. POLYDOROFF.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,200,263 Kramolin May 14, 1940 2,241,912 Kersten May 13, 1941 2,354,331 Polydorofl July 25, 1944 

